1. Field of the Invention
The present invention relates to a process for the adsorptive separation of low molecular weight hydrocarbons. In particular, the instant invention is directed to a process for separating propylene from propane and mixtures of low molecular weight hydrocarbons.
2. Description of the Related Art
The separation of propylene from low molecular weight hydrocarbon mixtures is an extremely important and large volume operation in the chemical and petrochemical industries. Catalytic cracking and steam cracking are among the most common and large scale processes leading to these mixed hydrocarbon streams. The need to recover propylene from propane-containing streams, in particular, is one of high economic significance in the synthesis of polypropylene elastomers. However, because of the close proximity in boiling points between propylene and propane, these components are presently separated by fractional cryogenic distillation. The large size of the columns and the energy intensity of this distillation process have, however, created large incentives for alternative means of effecting these separations in a more energy-efficient and cost-effective manner.
Some of the leading alternatives to fractional cryogenic distillation involve the use of porous materials that exploit their ability to selectively adsorb some of the components in the mixture. This has given rise to various forms of pressure or temperature swing adsorption (PSA/TSA) processes in which the mixture is first passed through an adsorbent material under conditions where one or more of the components are selectively removed. The loaded material is then typically exposed to a lower pressure and/or higher temperature environment where the adsorbed components are released and recovered at a higher purity level. Economic viability requires adsorbent materials that can deliver high selectivity, high adsorption capacity, and short duration cycles. An additional and critically important requirement is that the material should not catalyze chemical reactions that might lower the recovery of the desired components and/or render the adsorbent inactive.
Among the adsorbents which have been proposed for the recovery of propylene from hydrocarbon mixtures are ion exchange resins, mesoporous solids, activated carbons, and zeolites. Ion exchange resins and mesoporous solids usually exploit equilibrium adsorption properties in which one of the components is selectively adsorbed over suitably dispersed chemical agents. They principally rely on the adsorption affinity of cationic active centers, such as Ag and Cu, for the double bond in propylene (π-complexation). The characteristic time associated with the adsorption cycle is that required to bring the mixture close to thermodynamic equilibrium with the adsorbent. Because of the strong propylene-metal ion interaction, these systems generally require heat input to achieve rapid and complete propylene desorption. The relative rates of diffusion of the various components within the adsorbent are of secondary importance.
A second class of processes relies on the relative rates of diffusion within the adsorbent to carry out the separation. Two related cases are of interest here. In one extreme case, the separation is achieved by excluding the diffusion of some of the components into the adsorbent. This situation, in principle, leads to a maximum separation efficiency. The second case exploits a sufficiently large difference in diffusion rates that allow the preferential uptake of some of the components within a predetermined adsorption time. This case is commonly referred to as a kinetic-based separation scheme and is the method of choice to be used in conjunction with the materials disclosed in the present invention.
Activated carbons and zeolites typically resort to a combination of adsorption affinity and diffusion control. Carbons are usually activated to very high surface area forms in order to provide textural properties that simultaneously target the optimization of adsorption affinity and diffusion control. Using similar principles, zeolites have become even more attractive than activated carbons because of the ever-increasing possibilities afforded by new material synthesis procedures. Zeolites allow for a more flexible and precise control of critical properties such as chemical composition, internal surface area, pore volume, and window sizes. Chemical composition, internal surface area and pore volume are key variables controlling the adsorption capacity of the material. The tetrahedrally coordinated atoms, on the other hand, give rise to connecting windows of precise dimensions that control diffusional transport in and out of the crystallites.
Eight-membered ring zeolites, in particular, have been actively investigated for the separation of low molecular weight hydrocarbons because the window sizes of these zeolites are comparable to the molecular dimensions of low molecular weight molecules and because many afford high adsorption capacities. A typical example is the Linde type A zeolite that is characterized by a set of three-dimensional interconnected channels having 8-membered ring window apertures. The effective size of the windows depends on the type of charge-balancing cations. This has given rise to the potassium (3A), sodium (4A), and calcium (5A) forms, which have nominal window sizes of about 3 Å, 3.8 Å, and 4.3 Å, respectively. Thus, for example, EP-B-572239 discloses a PSA process for separating an alkene, such as propylene, from a mixture comprising said alkene and one or more alkanes by passing the mixture through at least one bed of zeolite 4A at a temperature above 323 K to preferentially adsorb said alkene and then desorbing the alkene from the bed. EP-A-943595 describes a similar process in which the zeolite adsorbent is zeolite A having, as its exchangeable cations, about 50% to about 85% sodium ions, about 15% to about 40% potassium ions and 0% to 10% of other ions selected from Group IA ions (other than sodium and potassium), Group IB ions, Group IIA ions, Group IIIA ions, Group IIIB ions and lanthanide ions.
In zeolites, it is well-accepted that the control of window size is important for achieving high separation selectivities. For a given zeolite structure type, the effective size of the windows can be sometimes further tuned by partially blocking or unblocking the windows with suitable charge-balancing cations.
In addition to window size control, an important requirement is that the adsorbent material should not catalyze any chemical reactions. This is particularly important for separating mixtures containing olefins, which can readily oligomerize on mildly acidic sites even at relatively low temperatures. Any residual catalytic activity of the adsorbent leading to detrimental reactions has to be avoided. These reactions not only lower the recovery of the desired components, but they are also likely to render the adsorbent inactive. The double bonds in the olefins, for example, are particularly prone to attack, even by mildly acidic centers and this may severely limit the temperature and partial pressures at which the separation process can be carried out.
In an effort to control chemical reactivity more reliably, there is a growing interest in the use of non-acidic, all-silica zeolites. Since these siliceous zeolites require no extra-framework balancing cations, the size of the windows is uniform and determined solely by the crystal structure. Thus, for example, the potential of deca-dodecasil 3R (DD3R) for separating propane and propylene has been very recently reported. See Zhu, W., Kapteijn, F., and Moulijn, J. A., “Shape Selectivity in the Adsorption of Propane/Propene on the All-Silica DD3R,” Chem. Commun. 2453-54 (1999). This crystalline microporous silicate has a two-dimensional pore system formed by 8-membered rings of tetrahedrally coordinated atoms with a nominal window size of 3.6 Å×4.4 Å (see Atlas of Zeolites Framework Types, Fifth Revised Edition, pages 108-109, 2001). Reported adsorption measurements on this material indicate that whereas propylene is able to diffuse to the interior of the crystallites, propane is largely excluded. However, the size of the DD3R windows appears to be so close to the effective kinetic diameter of propylene that the diffusion rates are very low, and this could lead to undesirably long adsorption and desorption cycles. Cycle duration can, in principle, be reduced by appropriate reductions in crystal size, but such changes are not always possible with the known synthetic procedures. Furthermore, the low dimensionality (2-D) and the high framework density (17.6 T/1000 Å3) suggest that DD3R has only a limited capacity for adsorbing propylene.
Relying on similar arguments of non-acidity, U.S. Pat. Nos. 6,293,999 and 6,296,688 disclose the use of AlPO-14 (AFN) for separating propylene from propane. Although AlPO-14 possesses a set of three-dimensional interconnecting 8-ring windows, only one of them is large enough to allow the passage of propylene; therefore, AlPO-14 effectively has a 1-dimensional diffusion system for hydrocarbons and it exhibits relatively low adsorption capacity. Moreover, with a nominal window size dimension of only 3.3 Å×4.0 Å (Atlas of Zeolites Framework Types, Fifth Revised Edition, pages 36, 37, 2001), the diffusion of propylene should be slow and associated with undesirably long adsorption cycles. Further, the above two patents reveal that AlPO-14 exhibits adsorption hysteresis, behavior that indicates that not all of the propylene is desorbed at low temperatures. This feature becomes dominant at lower temperatures; at 25° C. it reduces the effective reversible adsorption by ˜60% and thus decreases the effectiveness of this adsorbent.
U.S. Pat. No. 6,488,741 B2 teaches the use of two pure silica zeolites and one very high silica zeolite for the kinetic separation of propylene from propylene/propane mixtures. While the two pure silica zeolites, Si—CHA and ITQ-3, have desirable properties—i.e., high ratios of diffusion rate parameters and satisfactory adsorption capacities—they are very difficult and expensive to synthesize, thereby bringing their practicality into question.